Method for manufacturing a three dimensional circuit board

ABSTRACT

A molding pin for a metal die is prevented from breaking, solder is surely deposited, and thus, a circuit pitch can be reduced to the limit. On the front plane of a circuit board, prescribed circuit patterns made of a conductive material are formed, and on the rear plane, prescribed circuit patterns are also formed. On the circuit board, a through hole is formed to carry electricity between the circuit patterns on both planes. The inner shape of the through hole is narrow in a direction between the adjacent circuit patterns and wide in a circuit extending direction.

TECHNICAL FIELD

The present invention relates to a method for manufacturing a solid orthree-dimensional circuit board.

BACKGROUND ART

In recent years, demands for making much narrower intervals or pitchesbetween circuit patterns on a circuit board increase to make electronicdevices much smaller and to save resources and energy for theirmanufacturing. Accordingly, circuit patterns are formed on the bothsides of a circuit board, which are electrically connected by a throughhole of a conductive layer. Although the through hole is conventionallyformed by drilling, recently it is formed during the injection-moldingprocess for the circuit board itself (Patent Documents 1, 2, and 3).

Patent Document 1: JP unexamined patent application publication No.61-239694

Patent Document 2: JP unexamined patent application publication No.63-128181

Patent Document 3: U.S. Pat. No. 4,424,095

Structure of a conventional solid or three-dimensional circuit board 11will be described with reference to FIGS. 5 to 7. A circuit board 21 isprovided with circuit patterns 31, 41 on the both sides thereof, and thecircuit patterns are electrically connected by through holes 51. Thediameter of the through hole is in several-ten-micron order. The circuitboard having those through holes is formed by injection molding with amolding die having a circular-cylindrical pin implanted. The diameter ofthe through hole is several-ten-micron order, i.e. 30 to 50 microns. Theshape of the through hole 51 is cylindrical and has the same diameter inany points of its longitudinal direction. Ratio of the thickness of thecircuit board 21 to the internal diameter of the through hole 51 oraspect ratio is limited to about five to one (5:1). Namely, if theaspect ratio falls within 5:1, electroless plating can deposit, however,if the aspect ratio exceeds 5:1, deposit by electroless plating iseither difficult or impossible.

DISCLOSURE OF THE INVENTION Problem to be Solved by the Invention

In order to make much narrower intervals or pitches between the circuitpatterns for making electronic devices much smaller, the diameter of thethrough hole 51 has to be small in several-micron order, andconsequently and the aspect ratio has to be high. When the aspect ratiois high, a pin or projection in the injection molding die for formingthe through hole cannot withstand the flow pressure of a liquid resin inthe injection molding process, causing damages on the molding pinfrequently. Further, when the aspect ratio is high, deposit byelectroless plating is either difficult or impossible.

An object of the present invention is to provide a method formanufacturing a solid or three-dimensional circuit board, which canprevent the damage of a molding pin or projection in a molding die forforming a through hole of the circuit board, which can make electrolessplating to provide secure deposit, and which can make the pitch orinterval between the circuit patterns narrow to the limit.

Means for Solving the Problem

The first feature of the present invention is a method for manufacturinga solid or three-dimensional circuit board comprising the steps ofmolding a through hole onto a primary molded article of insulatingmaterial in a injection molding process with using a molding pin orprojection; molding a predetermined circuit pattern of conductivematerial on the both sides of a circuit board constituted of the primarymolded article in a plating process; forming a conductive layer over thethrough hole, which electrically connects the circuit patterns on theboth sides the circuit board. And in the method, the width of thethrough hole in the circuit-extended direction is larger than the widthof the through hole in the circuit-pattern-lined direction. Namely, theopenings of the through hole exposed in the circuit pattern are formedin a shape such that its width in the circuit-extended direction islarger than that in the circuit-pattern-lined direction, such asrectangular, elliptic, or the like.

The second feature of the present invention is a method formanufacturing a solid or three-dimensional circuit board according tothe first feature thereof, wherein the cross-section of the through holecut with a plane along the direction of the thickness of the circuitboard is a tapered shape. “Tapered shape” includes a shape widening orbroadening toward the end both in the circuit-pattern-lined directionand in the circuit-extended direction and a shape widening or broadeningtoward the end either in the circuit-pattern-lined direction or in thecircuit-extended direction.

The third feature of the present invention is a method for manufacturinga solid or three-dimensional circuit board according to either the firstor the second feature thereof, wherein a barrier is formed between theadjacent circuit patterns. When lead wire terminals are mounted onto thecircuit patterns by soldering, as the pitches or intervals between thecircuit patterns are extremely narrow, solder will splash, causingshort-circuit between the adjacent circuit patterns. The barrier isformed to prevent such solder-splash.

The fourth feature of the present invention is a method formanufacturing a solid or three-dimensional circuit board according toany of the first to the third features thereof, wherein the through holeis a blind through hole obtained by closing the through hole by plating.

Effect of the Invention

As the present invention effectively prevents the damage of a moldingdie, particularly damage of a molding pin or projection, and provides areliable deposit in plating, the pitches or intervals between theadjacent circuit patterns on a solid or three-dimensional circuit boardis cut down to the limit. Further, as the splash of solder is prevented,short-circuits between the adjacent circuits are avoided. Furthermore,as the blind through hole eliminates the step of cleaning flux in theprocess for mounting a circuit by soldering and prevents the outflow ofsolder, reliability of solid or three-dimensional circuit is enhanced.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plan view of a circuit board.

FIG. 2 is an enlarged sectional view taken along the line a-a in FIG. 1.

FIG. 3 is a bottom view of a circuit board.

FIGS. 4(A) to 4(F) are sectional views showing manufacturing steps.

FIG. 5 is a plan view of a conventional example.

FIG. 6 is a sectional view taken along the line a-a in FIG. 5.

FIG. 7 is a bottom view of a conventional example.

FIG. 8 is a plan view showing another embodiment.

FIG. 9 is a sectional view taken along the line b-b in FIG. 8.

FIG. 10 is a bottom view in another embodiment.

FIG. 11 is a planar top view of a molding pin of a molding die used inan injection molding process.

EXPLANATION OF REFERENCE NUMERALS

-   1 Solid/three dimensional circuit board-   2 Circuit board-   3 Circuit pattern-   4 Circuit pattern-   5 Through hole-   6 Barrier-   20 Primary molded article-   30 Circuit pattern-   40 Circuit pattern

BEST MODE FOR CARRYING OUT THE INVENTION

A structure of a solid or three dimensional circuit board 1 according tothe present invention will be described below with reference to FIGS. 1to 3. A circuit board 2 is made of a primary molded article of aninsulating material and is plated. One side of the circuit board 2 isprovided with a pre-determined circuit patterns 3, 3, . . . of aconductive material as shown in FIG. 1. The other side of the circuitboard 2 is provided with a pre-determined circuit patterns 4, 4, . . .of a conductive material as shown in FIG. 3. Through hole 5, 5, . . . ofconductive layer 5 a are formed on the circuit board 2, whichelectrically connect the circuit patterns 3, 3, . . . and 4, 4, . . . asshown in FIG. 2.

The through hole 5 is shaped such that its width in the circuit-extendeddirection is larger than that in the circuit-pattern 3, 3-lineddirection. The circuit-pattern 3, 3-lined direction means that adirection along which the circuit patterns 3, 3, . . . are lined up. Forexample, in FIG. 1, the through hole 5 is shaped in a rectangle with itsleft and right sides longer than its upper and lower sides. Such arectangular shape is not only applied to the upper-end opening 5 b ofthe through hole 5 but also applied to all through from there to thelower-end opening 5 c thereof as shown in FIGS. 2, 3. Namely, the widthof the through hole 5 in the circuit-extended direction is larger thanthat in the circuit-pattern 4, 4-lined direction. In FIG. 3, the throughhole 5 is shaped in a rectangle with its left and right sides longerthan its upper and lower sides. However, the through hole 5 is notlimited to a rectangular shape, and it may be a elliptical shape withits major axis along the circuit-extended direction. Therefore, as theshape of the through hole 5 is as such, the molding pin in a injectionmolding die for forming the through hole 5 is able to have a largerdiameter than the conventional pin, and the risk of damaging the pinconsiderably decrease.

The cross-section of the through hole 5 cut with a plane along thedirection of the thickness of the circuit board 2 is, as shown in FIG.2, a tapered shape widening toward the end from the upper-end opening 5b to the lower-end opening 5 c. As apparent in comparison between FIG. 1and FIG. 3, the size of the lower-end opening 5 c is about twice largerthan that of the upper-end opening 5 b. The width of the circuit pattern4 on the lower side of the circuit board 2 is about twice larger thanthat of the circuit pattern 3 on the upper side thereof, too. “Taperedshape” means, as shown in FIG. 2, that the through hole 5 widens orbroadens toward the end not only in the direction along which thecircuit patterns 3, 3, 4, 4 are lined up, but also in the directionalong which the circuit patterns 3, 4 are extended. Namely, the throughhole 5 widens or broadens toward the end not only in the lateraldirection of FIGS. 1, 3 but also in the longitudinal direction of FIGS.1, 3. However, it dose not mean that the through hole 5 has to widen orbroaden toward the end in the both directions. The through hole 5 maywiden or broaden toward the end either in the circuit-pattern 3, 3, 4,4-lined direction or in the circuit-pattern 3, 4-extended direction.

By adopting the tapered through hole 5, even though an apparent aspectratio is about 1:10, the substantial aspect ratio is limited to 1:5.Thereby, catalyst solution or an electroless solution is smoothlycirculated in the through hole to ensure catalyst-deposit andplating-deposit. As the molding pin in an injection-molding die forforming the through hole 5 is formed in a trapezoidal section shape,risk of damaging the molding pin reduces considerably.

Further, recently, the circuit patterns 3, 4 are formed such that thepitch or interval between the adjacent circuit patterns is extremelynarrow to the limit. Therefore, when lead line terminals are mountedonto the circuit patterns by soldering, solder will splash, causingshort-circuits between the adjacent circuit patterns 3, 3. In order toprevent such solder-splash, a barrier 6 is formed between the adjacentcircuit patterns.

Steps in manufacturing a solid/three dimensional circuit board accordingto the present invention is described below with reference to FIGS.4(A)-(F). FIG. 4(A) shows a primary molded article 20, which is moldedby injecting a plating-grade liquid-crystal polymer into a closedmolding die. The outer shape of the primary molded article 20 is matchedwith the circuit board 2 serving as a final product. The through hole 5is formed on the primary molded article 20 by a molding pin 52 (see FIG.11) in the injection molding process. Aromatic polyester is used as theliquid crystal polymer.

The primary molded article 20 is subjected to a plating process.However, before the plating process, the upper side of the primarymolded article 20 is first chemically etched to form a coarsened surface20 a as shown in FIG. 4 (B) The etching process is performed as follows.An alkaline aqueous solution is obtained by solving sodium hydroxide orpotassium hydroxide in water at a pre-determined concentration and isheated to a pre-determined temperature. The primary molded article 20 isdipped in the alkaline aqueous solution for a pre-determined period oftime.

Thereafter, the primary molded article 20 is inserted into anothermolding die, whose cavity is shaped such that a pre-determined gap ismade between its inner surface and the primary molded article 20 whenthe primary molded article 20 is inserted. After closing the moldingdie, oxyalkylene-group-containing polyvinyl alcohol resin such as“ECOMATY AX”, a product of Nippon Synthetic Chemical Industry Co., Ltd.,is injected into the molding die for masking the primary molded article20. Thereby, a secondary molded article 200 having a mask 7 on thepre-determined portion of the upper side thereof is formed as shown inFIG. 4(C).

Next, a catalyst 8 consisting of palladium or gold is applied onto acoarsened surface 20 a of the secondary molded article 200 as shown inFIG. 4(D). The catalyst application may be conducted by publicly knownmethods such that palladium is deposited on the surface of the secondarymolded article 200 by dipping it in a silver- and palladium-based mixedcatalyst solution and by activating it with acid such as hydrochloricacid or sulfuric acid. Another method is that a relatively strongreducer such as stannous chloride is adsorbed on the surface of thesecondary molded article 200 and then it is dipped in a catalystsolution containing noble metal ions such as gold ions. In this process,as the through hole 5 is tapered as described above, the catalystsolution is smoothly circulated to reliably deposit gold on the surface.

Next, the secondary molded article 200 is heated in hot water and themask 7 molded by secondary molding is solved into the hot water as shownin FIG. 4(E). Namely, when the secondary molded article 200 is put inhot water at 80° C. for 10 minutes, “ECOMATY AX” completely solves inthe hot water.

Then, the circuit patterns 3, 4 are formed on the catalyst-appliedportion of the secondary molded article 200 by plating, as shown in FIG.4(F). In this step, as an electroless solution smoothly circulates andflows in the through hole due to its tapered shape as described before,plating can be securely formed. For plating, either chemical copperplating, chemical nickel plating, or the like is used.

Finally, heat treatment is performed to remove moisture in the secondarymolded article 200 to finish the step of forming a conductive circuit,thereby the solid/three dimensional circuit board 1 is completed.

In the above embodiment, however, disadvantage is observed in a casesuch that an electronic element is mounted on the circuit pattern 3 ofthe one side of the circuit board 2 by soldering and the circuit pattern4 of the opposite side thereof is used as an electric contact. Eitherflux or solder passes from the one side of the circuit board 2 throughthe through hole 5 and flows onto the circuit pattern 4 of the oppositeside thereof, thereby damaging the function of circuit pattern 4 as anelectric contact.

Therefore, another embodiment will be described below with reference toFIGS. 8 to 10. The through hole 5 is closed with a plating metal 30 andthe interior of the plating metal 30 is formed in a cone-shaped hollow.The lower end of the plating metal 30 serves as a circuit pattern 40 onthe lower side of circuit board 21. The rest of the configuration ofthis embodiment is the same as that of the previous embodiment, andaccordingly the same reference numerals are applied. In this embodiment,the step of cleaning flux in the process for mounting a circuit bysoldering is eliminated and an outflow of solder is prevented.Therefore, reliability of the solid/three dimensional circuit isimproved.

INDUSTRIAL APPLICABILITY

The present invention is applied to such a example as a connector whichelectrically connects, via a coaxial cable, the electric circuit portionand liquid crystal display portion of a portable cellular phone, both ofwhich are pivotally jointed together.

The invention claimed is:
 1. A method for manufacturing athree-dimensional circuit board, comprising the steps of: forming athrough hole onto a primary molded article of insulating material with amolding pin or projection during an injection molding process; formingpredetermined circuit patterns of a conductive material, respectively,on both sides of a circuit board constituted of the primary moldedarticle in a plating process, wherein at a given side of the circuitboard the predetermined circuit patterns are lined up with each one ofsaid given side predetermined circuit patterns extending in a firstdirection; and forming a conductive layer over the through hole, whichelectrically connects the respectively formed predetermined circuitpatterns on the both sides of the circuit board; wherein said throughhole is polygonal and tapered in a direction extending from one side toanother side of said both sides, wherein for every depth level of thethrough hole from one side to said another side of the circuit board, aplanar area of the polygonal through hole at each one depth level has afirst dimension larger than a second dimension orthogonal to the firstdimension; wherein at said given side of said both sides of the circuitboard, a shape of the through hole has said first dimension in saidfirst direction that is larger than said second dimension along saidgiven side, said second dimension being orthogonal to the firstdimension.
 2. A method for manufacturing a three-dimensional circuitboard according to claim 1, wherein a barrier is formed between adjacentcircuit patterns within said respectively formed predetermined circuitpattern formed on said given side, said circuit board having a thicknessat said barrier greater than at an edge of said through hole adjacentsaid barrier for preventing solder splash during said formingpredetermined circuit patterns.
 3. A method for manufacturing athree-dimensional circuit board according to claim 1, wherein thethrough hole is blinded by closing the through hole by plating.
 4. Amethod for manufacturing a three-dimensional circuit board according toclaim 1, wherein a barrier is formed between adjacent circuit patternswithin said respectively formed predetermined circuit pattern formed ona first side of said both sides.
 5. A method for manufacturing athree-dimensional circuit board according to claim 2, wherein thethrough hole is blinded by closing the through hole by plating.
 6. Amethod for manufacturing a three-dimensional circuit board according toclaim 1, wherein a barrier is formed between adjacent circuit patternswithin said respectively formed predetermined circuit pattern formed onsaid given side, said barrier comprising a region of greater elevationrelative to adjacent regions to each side of the barrier on the givenside.